Method for developing latent electrostatic images

ABSTRACT

THIS INVENTION RELATES TO A METHOD FOR DEVELOPING AN ELECTROSTATIC LATENT IMAGE ON AN IMAGE SUPPORT WHICH COMPRISES CONTACTING THE IMAGE WITH A FUSIBLE RESIN POWDER TO DEVELOP THE IMAGE AREAS AND THEN CONTACTING THE IMAGE AND SUPPORT WITH A MIXTURE OF EFFECTIVE AMOUNTS OF FINELY DIVIDED IRON CARRIER PARTICLES AND A POWDER WHICH REMOVES UNDESIRED FUSIBLE RESIN PARTICLES FROM THE NON-IMAGE AREAS   OF THE SUPPORT, THEREBY PRODUCING A DEVELOPED IMAGE WITH CLEAN NON-IMAGE AREAS.

July 13, TUNG-NAN HENG METHOD FOR DEVELOPING LATENT ELECTRQSTATIC IMAGESFiled Oct. 9, 1967 DEVELOPED POWDER DEVELOPED POWDER zmc OXIDE COATEDIMAGE WITH SOME IMAGE WITH CLEANER ELECTROPHOTOGRAPHIC BACKGROUNDBACKGROUND q PAPER i--- -t I NEGATIVELY CHARGED T REGULATOR LATENTELECTROSTATIC Wv Dc. VOLTAGE IMAGE IMAGE DEVELOPER WITH CYLINDRICALCLEANING DEVELOPER POSITIVELY CHARGED TONER MAGNETIC CDMPOSED 0FALUMINUM AND IRDN PARTICLE CARRIER BRUSHES SULFATEAND IRON PARTICLECARRIER INVENTOR TUNG-NAN CHENG BY I Z- ATTOR Y United States PatentOflioe 3,592,675 METHOD FOR DEVELOPING LATENT ELECTROSTATIC IMAGESTung-Nan Cheng, Bloomfield, N.J., assignor to Azoplate Corporation,Murray Hill, NJ. Filed Oct. 9, 1967, Ser. No. 673,600 Int. Cl. G03g13/08 US. Cl. 117-175 11 Claims ABSTRACT OF THE DISCLOSURE The tonerscommonly used in electrophotography comprise natural or synthetic resinsin finely divided powder form. When used for the development of latentelectrostatic images, such a toner is mixed with somewhat largerparticles of a carrier which generally comprises an inorganic material,e.g. iron powder.

Iron powder is particularly effective as a carrier as it can be movedmagnetically, for example, by means of a roller magnetized by electriccurrent. The toner present on the surface of the iron powder is thusapplied in the desired manner to the surface of the electrostatic imageto be developed. Thus, an even application of toner to fairly largesolid areas is possible.

It has been suggested to use toners consisting of two components whichhave approximately the same particle size. One toner component acquiresa positive charge during handling and the other toner component acquiresa negative charge. The use of these toners gives particularly desirableresults with respect to freedom from background and sharpness of imagesproduced therewith.

Although the two component toner system provides developed electrostaticimages with cleaner non-image areas than does the single component tonersystem background still remains.

The present invention provides a method for developing latentelectrostatic images in which copies are obtained having much cleanernon-image areas.

In the present invention, a latent electrostatic image is firstdeveloped with a single component or double component toner using any ofthe known methods of development, such as using the toner in admixturewith an iron particle carrier and applying the mixture to the image tobe developed by means of a magnetic roller or magnetic brush.Alternatively, the image may be developed by cascade development, powdercloud development, or sprinkling the toner on the latent image. Thedeveloped electrostatic image, having some toner adhering in thenonimage areas, then is passed in contact with one or more magneticrollers containing another powder in admixture with an iron particlecarrier. The developer powder on the second magnetic roller picks up theweakly or loosely bonded colored toner in the non-image areas whileleaving 3,592,675 Patented July 13, 1971 the strongly bonded coloredtoner in the image areas. The developed image is then fixed by heat.

The accompanying drawing shows one embodiment of the method of thepresent invention in which a zinc oxide electrophotographic paper havinga negatively charged latent electrostatic image thereon is passed incontact with an image developer comprising a positively charged toner inadmixture with an iron particle carrier and the developed powder image,with some toner particles in the non-image areas, is passed in contactwith a cleaning developer comprising aluminum sulfate, for example, theadmixture with iron particles, the mixture being applied by means of amagnetic brush to produce a developed powder image with clean non-imageareas. A bias voltage may be applied for example to the magnetic brushused to apply the cleaning developer to the developed powder image. Ofcourse, the zinc oxide electrophotographic paper can be any other knownelectrophotographic material such as metallic bases, including aluminum,copper, and the like, having inorganic or organic photoconductorsthereon or the base may be paper having photoconductors other than zincoxide, such as the other known inorganic and organic photoconductivematerials. The development of the latent electrostatic image and theapplication of the cleaning developer to the developed image isperformed in the dark.

Among the fusible resin powders which may be employed to develop theimage areas are the known resins, such as natural and synthetic resins,including colophony, copals, dammar resin, asphalts, colophony-modifiedphenol resins, polyacrylic acid resins and polystyrenes, as well asmixtures of these resins. Inorganic and/ or organic pigments anddyestuffs may be added to these resins to impart a definite polaritythereto. Examples of suitable substances are carbon black, zinc oxide,titanium dioxide, barium sulfate, minium and dyestuffs such as thoselisted in Schultzs Farbstolftabellen, vol. I, 7th edition (1931). Also,mixtures of such dyestuffs and/ or pigments may be used.

Minute quantities of these substances, e.g. 0.5 percent by weight ofcarbon black, are often sufficient to impart a polarity to the resins.It is also advantageous to incorporate small quantities of waxes and/ ororganic compounds of low melting point and their substitution products,e.g. 0.1 to 10 percent, preferably 1 to 5 percent by weight, to thetoner component to influence in a desirable direction its melting pointand adhesive power.

Waxes suitable for this purpose include natural waxes such as carnaubawax, beeswax, Japan wax, montan wax, ceresine and synthetic waxes suchas those marketed by A-wax, OP-wax, SPO-Wax, V-wax, O-wax, E-wax, HardWax H, Hard Wax W, various Waxes known as Ruhrwachse, and particularlythe products available as Gersthofener waxes with the codings S, L, O,and OP.

Suitable organic compounds of low melting point in clude, in particular,substituted and unsubstituted aromatic compounds having melting pointsbetween 40 and C., for example naphthols, such as l-naphthol and2-naphthol, aromatic compounds such as acenaphthalene, acylaminocompounds such as acetanilide, halogenated aromatic compounds, such asp-dibromobenzene, amino compounds such as 2,4-diamino toluene,o-phenylene diamine, phenols such as resorcinol, and diphenylamine andderivatives thereof.

The powder which is admixed with finely divided iron carrier particlesand is used as a cleaning developer in the present invention may beorganic or inorganic. Exemplary of such powders are those listed in thetable below, which table also shows the optimum concentration of thepowder in admixture with finely divided iron carrier particles and alsothe triboelectric polarity of the powder:

Optimum concentration (precent 'Iriboeleetrie byweight) polarityKieselguhr 3-15 Magnesium trisilioat 5-20 Aluminum suliate 5-25 Aluminumstearate 5-20 Asbestos 5-20 Calcium carbonate 5-20 Ferric sulfate 5-20Zine carbonate Sodium phosphate, dibasic 5-25 Ethylenedinitrilo tetraacetic ac 5-20 Zinc stearate 0- Sodium oleato 5-25 Totrahdrophthalicanhydride 5-20 Diplienyl phthalate 5-20 g) Para-phenolsulionic sodiumsalt 5-20 E) Gallic acid- 5-20 Benzoic acid 5- 0 Para acetophenetidine.5-20 5-) Zinc oxide 5-20 Benzo henone tetraearboxylie dian iydridecharged. Both polarities are observed.

The finely divided cleaning developer powders in admixture with ironcarrier particles form a soft layer around the magnetic tubes of themagnetic brush so that it does not scratch the unfixed powder image.

The triboelectric characteristics of the finely divided powders whenadmixed with an iron carrier also consti tute a factor in the powderselection. For example, if the colored toner used for developing theimage is positive in polarity then the finely divided powder used in thecleaning step should preferably be negative in polarity, and vice versa.However, finely divided powders which show both polarities when mixedwith iron carrier particles, or mixtures of two or more different finelydivided powders also are useful.

The reasons why a finely divided powder has both positive and negativepolarities when mixed with iron carrier particles is not fullyunderstood but may be due to oxidation, one or more organic residues,moisture, dust or other contaminants.

The developer on the first magnetic roller shown in the drawing used todevelop the latent image may contains, as is known, one part of toner to10 to parts of iron particles by weight. Where a double toner is used,the ratio of the fusible toner to the inorganic toner, such askieselguhr, may vary from about 1 to l to 1 to 10, by weight.

For the developer on the second magnetic roller, the ratio of thecolorless toner, to the iron carrier particles may vary from 1 to 4 to 1to 200 preferably from 1 to 5 to l to 200 by weight. The preferredparticle size of the iron carrier particles is from to 200g.

The advantages of using a single component colored toner followed by thecleaning action of the magnetic roller in the present invention, asopposed to the use of a double toner applied by a single magneticroller, is that there is no adhesion of the cleaning toner, such askieselguhr, to the non-image areas. This eliminates the step of cleaningthe kieselguhr from the surface of the final copy and, also, more evenimage areas can be obtained, especially on large solid areas.

The bias voltage may be applied, depending upon the nature of the toner,to either or both magnetic rollers shown in the drawing to obtain theoptimum results. The

bias voltage may vary within a wide range depending upon the method ofapplication. For a cleaning device in cluding an electrically conductivemagnetic brush and a non-magnetic conductive container, the preferredrange is between 200 and 150 volts. DC, however, lower or highervoltages may be used. The connection of the power source to the magneticbrush depends upon the polarity of the finely divided powder in thecleaning developer. For example, if the finely divided powder isnegative in polarity then the magnetic brush preferably is connected tothe positive terminal, the negative terminal being grounded, and viceversa. For safety, the support of the photoconductive coating is to begrounded if a bias voltage is applied.

The rotation of the magnetic brush which applies the cleaning developercan be in either direction with respect to the direction of travel ofthe photoconductive surface shown in the drawing, but the surface speedof the rotaing cleaning magnetic brush should be greater than the speedof travel of the photoconductive surface if it is in the same direction.

The invention will be further illustrated by reference to the followingspecific examples:

EXAMPLE 1 A zinc oxide coated electrophotographic paper is negativelycharged by means of a corona discharge and then exposed to a lightimage. The negatively charged latent image is developed with apositively charged colored toner in admixture with an iron particlecarrier by means of a magnetic brush. The resulting developed imageshows some toner or background in the non-image areas, as is customary.

The electrophotographic paper with the powder image and some backgroundis then brought into surface contact with the cleaning device shown inthe drawings which consists of a. rotating magnetic brush and a cleaningdeveloper composed of 15 percent of finely divided aluminum sulfate and85 percent of finely divided iron particles, by weight, whereby nearlyall of the toner in the non-image areas is removed without loss of imagedensity. The powder image is then fixed by application of heat, in knownmanner.

EXAMPLE 2 The procedure of Example 1 is followed except that a basisvoltage of 50 volts DC is applied to the cleaning device shown in thedrawing with the positive terminal connected to the cleaning device andthe negative terminal and the support of the photoconductive coatingconnected to ground. The cleaning device is insulated from the remainderof the machine.

EXAMPLE 3 An organic photoconductive substance and a binder aredissolved in a solvent and coated on an aluminum substrate. Thephotoconductive surface is positively charged by means of a coronadischarge. The photoconductive aluminum plate is then exposed to a lightimage. The positively charged latent electrostatic image then isdeveloped with a negatively charged colored toner, using beads as acarrier, by means of the cascade developing method. The developed powderimage with some toner particles in the non-image areas is then broughtinto surface contact with the cleaning device shown in the drawingsconsisting of a magnetic brush and a cleaning developer which iscomposed of 16 percent of aluminum stearate and 85 percent of ironparticles, by weight. An electrostatic image with much cleaner non-imageareas is obtained. The powder image is then fixed by application ofheat, in known manner.

EXAMPLE 4 The procedure of Example 3 is repeated except that a biasvoltage of volts DC is applied to the cleaning device, which latter isinsulated from the remainder of the machine. The negative terminal isconnected to the cleaning device and the positive terminal and thealuminum support of the photoconductive coating is grounded.

EXAMPLE A zinc oxide coated electrophotographic paper is processedaccording to Example 1 wherein a cleaning developer composition is usedconsisting of percent by weight of tetrahydrophthalic anhydride and 90percent by weight of finely divided iron particles. This composition isprepared by first grinding the tetrahydrophthalic anhydride to aparticle size between 0.5 and p. and then intimately mixing with theiron powder.

EXAMPLE 6 A Zinc oxide coated electrophotographic paper is processedaccording to Example 1 wherein as a cleaning developer a mixture of anorganic compound, in this case 5 percent by weight ofpara-acetophenetidine and an inorganic compound, in this case 5 percentby weight of aluminum sulfate, with 90 percent of finely divided ironparticles by weight is used.

The method of the present invention also can be applied to transferelectrophotography, i.e., xerography. In the xerographic process, axerographic drum comprising a layer of photoconductive insulatingmaterial, such as selenium, on a conductive backing is given a uniformpositive electrostatic charge over its surface and then exposed to alight image, thereby forming an electrostatic latent image on thephotoconductive layer.

Development of the latent image is effected with a negatively chargedcolored toner brought into surface contact with the photoconductivelayer. Thereafter, the developed powder image is transferred to asupport to which it may be fixed by application of heat. However, duringthe cascade developing method employed, quite often the electricallycharged colored toners adhere to the nonimage areas and result in copieshaving undesirable background. It is, accordingly, advantageous tocontact the powder image immediately after cascade development, butbefore the transfer of the powder image to a support surface, with thecleaning device of the present invention.

Since the photoconductive insulating layer, such as selenium, isinitially positively charged, a negatively charged colored toner isemployed. Therefore, the finely divided powder in the cleaning steppreferably is positive in polarity when mixed with iron particles.However, as noted above, it is also possible to use powders whichexhibit both polarities, or a mixture of two or more dilferent finelydivided powders. Alternatively, the cleaning step of the presentinvention can be performed on a powder image that has been transferredbefore the image has been fixed. In this case, it is preferable toconstruct the system in a manner such that the unfixed powder imagefaces downwardly when it passes over and in surface contact with thecleaning magnetic brush. A basis voltage may be applied advantageouslyto the magnetic brush of the cleaning device when used both aftercascade development or after the image has been transferred.

Another application of the cleaning device can best be described as aweb cleaner. In transfer electrophotography, i.e., xerography, aphotoconductive cylindrical drum is continuously rotated through a cycleof sequential operations, including charging, exposure, developing, andtransfer, resulting in multiple copies; the photoconductive surface iscleaned by means of the present invention before repeating the cycle. Toaccomplish this, the residual toner on the photoconductive surface,after transfer of the powder image to the support surface, particularlyin the image areas, is first substantially neutralized, then broughtinto surface contact with the cleaning device, and finally brushed witha rotating fur brush whereby all residue is collected by vacuum anddeposited into a filter bag.

It will be obvious to those skilled in the art that many modificationsmay be made within the scope of the present invention without departingfrom the spirit thereof, and the invention includes all suchmodifications.

What is claimed is:

1. A method of developing an electrostatic latent image on an imagesupport which comprises contacting the image with a fusible resin powderto develop the image areas and then contacting the developed image andsupport with a magnetic brush containing a mixture of effective amountsof finely divided iron carrier particles and a second powder whichremoves undesired fusible resin particles from non-image areas of thesupport, whereby a developed image with clean non-image areas isobtained.

2. A method according to claim 1 in which the second powder isinorganic.

3. A method according to claim 1 in which the second powder is organic.

4. A method according to claim 1 in which the second powder comprises amixture of inorganic and organic powders.

5. A method according to claim 1 in which the second powder is ametallic salt of an organic acid.

6. A method according to claim 1 in which the second powder is selectedfrom the group consisting of aluminum sulfate, aluminum stearate, tetrahydrophthalic anhydride, diphenyl phthalate, para 'acetophenetidine and'b-enzophenone tetracarboxylic dianhydride.

7. A method according to claim 1 in which the second powder has apolarity opposite to that of the fusible resin powder. i

8. A method according to claim 1 in which the second powder has anegative polarity.

9. A method according to claim 1 in which the second powder has apositive polarity.

10. A method according to claim 1 in which the second powder has both apositive and a negative polarity.

11. A method according to claim 1 in which a bias voltage is applied tothe mixture of iron carrier particles and second powder.

References Cited UNITED STATES PATENTS 2,919,247 12/ 1959 Allen 252-62.12,959,153 10/1960 Hider 117-17.5 3,272,644 9/ 1966 Nelson 117-1753,320,169 5/1967 East et al. 117-17.5 3,262,806 7/1966 Gourg 118-6373,103,445 10/1963 Bogdonoif et al. 117-17.5 3,318,284 5/1967 Hojo et al117-175 3,357,402 12/1967 Bhagat 117-175 3,392,139 7/1968 Dingman117-175 3,411,932 11/1968 Malone et al 117-19 3,424,615 1/ 1969 Eichornet al. 134-7 2,772,991 12/1956 Insalaco 1347 2,484,782 11/ 1949 Copley134-7 WILLIAM D. MARTIN, Primary Examiner M. SOFOCLEOUS, AssistantExaminer U.S. Cl. X.R.

